Combustor chamber cooling

ABSTRACT

A gas turbine engine combustor which is cooled by a hybrid cooling apparatus and method. Heat shields cooled by impingement cooling air are present at an upstream section of the combustor liners and exhausted impingement cooling air is substantially discharged outside of the primary zone of the combustor chamber. The single-skinned downstream section of the combustor liners is cooled by effusion cooling.

TECHNICAL FIELD

The application relates generally to gas turbine engines and, moreparticularly to cooling a combustor for such engines.

BACKGROUND

In aviation gas turbine engines, the amount of air supplied forcombustion and dilution may be optimized for operability and to minimizeemissions such as nitrous oxide (NOx), carbon monoxide (CO),hydrocarbons (HC), etc. Therefore, it is often desirable that the amountof air supplied for cooling combustor walls be minimized, which poseschallenges to meeting the durability requirements of the combustorwalls, because the reduction in combustion wall cooling air may lead tounwanted material oxidation, thermal mechanical fatigue and/or thermalwall buckling due to thermal gradients. Particularly in small aero gasturbine engines, the total amount of air available for combustor wallcooling within the gas turbine thermodynamic cycle can be limited,especially where rich-burn combustion is sought. Therefore it is achallenge to optimize the combustor wall cooling while still meeting thedurability requirements of the combustor.

SUMMARY

There is provided a combustor for a gas turbine engine comprising: innerand outer liners spaced apart from each other to define a chambertherebetween including a primary zone adjacent an upstream end of thechamber and a secondary zone downstream of the primary zone, theupstream end being closed by a dome, a dome heat shield disposed withinthe chamber, adjacent and spaced apart from the dome, an inner linerheat shield disposed within the chamber, adjacent and spaced apart fromthe inner liner, an outer liner heat shield disposed within the chamber,adjacent and spaced apart from the outer liner, the inner and outerliner heat shields extending from the upstream end of the chamber overthe primary zone and terminating at the secondary zone, the inner andouter liners and the inner and outer liner heat shields defining aplurality of apertures therein for introducing dilution air jets intothe chamber, the combustor including a plurality of impingement holesdefined in the dome and inner and outer liners for directing impingementcooling air therethrough and into the combustor to impinge on a coldside of the respective heat shields, and a discharging apparatus todirect exhausted impingement cooling air discharged from the cold sideof the respective heat shields to flow along and substantially parallelto a hot side of the respective inner and outer liner heat shields, thecombustor further including a plurality of effusion holes defined in asection of the inner and outer liners free of coverage by the respectiveinner and outer liner heat shields, the effusion holes directingeffusion cooling air therethrough and into the combustor to effusioncool said section of the inner and outer liners.

There is also provided a method for hybrid cooling a combustor of a gasturbine engine comprising steps of: a) providing heat shield panels in achamber of the combustor to protect respective upstream sections ofinner and outer liners of the combustor, the upstream sections of theinner and outer liners substantially covering a primary zone of thechamber; b) directing impingement cooling air for impingement cooling acold side of the heat shield panels and then discharging a first portionof the impingement cooling air along and substantially parallel to a hotside of the heat shield panels and a second portion of the impingementcooling air into areas downstream of the heat shield panels, to therebyhave the discharged cooling air substantially outside of the primaryzone in order to reduce emission formation; and c) directing cooling airinto a downstream area of the chamber for effusion cooling of respectivedownstream sections of the inner and outer liners free of coverage bythe heat shield panels.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a schematic side cross-sectional view of a gas turbine engine;

FIG. 2 is a chart illustrating a relationship between flame temperatureand fuel air ratio of gas turbine engines;

FIG. 3 is a partial cross-sectional view of the combustor of the gasturbine engine of FIG. 1, illustrating cooling arrangements of thecombustor according to one embodiment of the present disclosure; and

FIG. 4 is an enlarged portion of the combustor of FIG. 3, showing anupstream end of the combustor chamber with detailed features.

It will be noted that throughout the appended drawings, like featuresare identified with like reference numerals.

DETAILED DESCRIPTION

FIG. 1 illustrates an aircraft turbofan gas turbine engine presented asan example of the application of the described subject matter, includinga housing or nacelle 10 including an annular core casing or engine outercase 13, a low pressure spool assembly seen generally at 12 whichincludes a fan assembly 14, a low pressure compressor assembly 16 and alow pressure turbine assembly 18, and a high pressure spool assemblyseen generally at 20 which includes a high pressure compressor assembly22 and a high pressure turbine assembly 24. The annular outer case 13surrounds the low and high pressure spool assemblies 12 and 20 in orderto define a main fluid path (not numbered) therethrough. A combustor 26is provided in the main fluid path. The combustor 26 according to thisembodiment is a reverse-flow type as illustrated. However, the subjectmatter described below is also applicable to a combustor of astraight-flow type.

In small aero gas turbine engine combustors, the total amount of airavailable for combustor wall cooling is very limited especially withrich burn technologies. To improve small aero engine Specific FuelConsumption (SFC), the pressure drop across the combustor wall has to bereduced below 3.0%, whereas in larger engines a pressure drop >3.0%across respective inner and outer liners and >4.5% pressure drop in adome panel bulkhead due to optimal wall cooling may be possible. The SFCof larger engines can be improved by other means, such as ultra highpressure ratio and active turbine tip clearances etc., which are moreaffordable due to engine size.

In smaller engines, however, to improve emissions more mass flow isoften desirable for combustion. Due to various constraints such ascompressor mass flow, pressure ratios, combustor delta pressure, and arelatively larger combustor surface area for cooling, the wall coolingcan be optimized in order to increase mass air flow for the combustionnecessary to improve emissions.

To minimize NOx formation and to reduce the flame temperature in aprimary zone of the combustor for high or medium pressure ratio engines,a high Fuel Air Ratio (FAR>0.1, Stoichiometric FAR=0.068) is desirable.As illustrated in FIG. 2, these pockets of high Fuel Air Ratio (FAR) ina primary zone and advance effusion cooling with coolant, would reducethe FAR toward the stoichiometric (as indicated by circles A and B inFIG. 2) and would stabilized the flame near the wall. This will increaseNOx formation due to the high local gas temperatures at close to thestoichiometric FAR. Carbon monoxide (CO) and hydrocarbon (HC) formationcan be reduced if the wall temperature in the primary zone is keptrelatively high. Circle C in FIG. 2 indicates a Fuel Air Ratio (FAR)operating zone of low pressure ratio engine combustors. Additionaleffusion coolant would decrease the local FAR and move same away fromthe stoichiometric region, as indicated by circle D in FIG. 2, therebyfurther reducing the flame temperature.

Therefore, to optimize the combustor cooling to improve emissionreduction and durability of the combustor, the combustor may be providedwith hybrid cooling techniques using heat shields in an upstream end ofthe combustor chamber with impingement cooling and having exhaustedimpingement cooling air substantially outside the primary zone in thechamber, meanwhile using effusion cooling for the single-skin wall of adownstream section of the combustor chamber which includes therein thesecondary zone.

According to one embodiment of the present disclosure as shown in FIG.3-4, the combustor 26 which may be used with rich burn techniques for agas turbine engine of high/medium pressure ratio, includes annular innerand outer liners 30, 32 spaced apart from each other to define anannular chamber 28 therebetween. The annular chamber 28 may have astraight upstream section with an upstream end closed by a bulkhead ordome 34. The dome may be formed with a plurality of dome panels. Theannular chamber 28 may include a downstream section in a curved shape(as in a reverse type of combustor) with an open downstream end fordischarging combustion gases to power the turbines 24, 18 of the engine.A plurality of circumferentially spaced apart fuel nozzles 36 withswirlers 38 are attached to the dome 34 for introduction of fuel and airinto the chamber 28 for combustion. Combustion in the chamber 28 mayprincipally take place in the primary zone 40, adjacent the upstream endof the chamber 28, and the combustion reaction may then continue in thesecondary zone 42 downstream of the primary zone 40.

A dome heat shield 44, which may be formed with a plurality of dome heatshield panels, is disposed within the chamber 28, adjacent and spacedapart from the dome 34, attached to the dome 34 to protect the dome fromexposure to the hot combustion gases in the chamber 28. An inner linerheat shield 46 which may be formed with a plurality of inner liner heatshield panels, is disposed within the chamber 28, adjacent and spacedapart from the inner liner 30, attached to the inner liner 30 to protectthe inner liner 30 from being exposed to the hot combustion gases withinthe chamber 28. An outer liner heat shield 48 which may be formed with aplurality of outer liner heat shield panels, is disposed within thechamber 28, adjacent and spaced apart from the outer liner 32, and isattached to the outer liner 32 to protect the outer liner from beingexposed to the hot combustion gases within the chamber 28. The inner andouter liner heat shields 46 and 48 extend from the upstream end of thechamber 28 over the primary zone 40 and terminate at the secondary zone42 and therefore are used only to protect the upstream sections of theinner and outer liners 30 and 32. Each of the dome heat shield 44 andinner and outer liner heat shields 46, 48 may include a plurality of pinfins 49 projecting from a cold side thereof (facing the respective dome34, inner and outer liners 30, 32) to increase contact areas withcooling air to increase heat transfer in convection cooling.

The inner and outer liners 30, 32 and the inner and outer liner heatshields 46, 48 define a plurality of apertures therein which are incommunication with the chamber 28, for introducing dilution air jets 52into the chamber 28 between the primary zone 40 and the secondary zone42. The dilution air helps to reduce flame temperature by quenching inthe secondary zone 42 and provides for combustor exit temperaturedistribution acceptable for turbines 24, 18.

A plurality of impingement cooling holes 54 (see FIG. 4) may be providedin the respective inner and outer liners 30, 32, for example at arelatively upstream section, as well as in the dome 34 for directingimpingement cooling air therethrough and into the combustor 26 toimpinge on the cold side of the respective dome heat shield 44 and linerheat shields 46 and 48. The impingement cooling air flows in the spacebetween the dome 34 and the dome heat shield 44, between the inner liner30 and the inner liner heat shield 46 and between the outer liner 32 andthe outer liner heat shield 48, contacting the cold side of therespective heat shields 44, 46, 48 and the pin fins 49, thereforeconvection cooling also takes place.

A discharging apparatus may be provided for directing exhaustedimpingement cooling air discharged from the cold side of the respectiveheat shields to flow substantially along and parallel to a hot side(facing the hot combustion gases in the chamber) of the respective heatshields. For example, the dome shield 44 is positioned with respect tothe inner and outer liner heat shields 46, 48 to provide respective gaps56, 58 between the dome heat shield 44 and the inner liner heat shield46, and between the dome heat shield 44 and the outer liner heat shield48. The dome heat shield 44 is configured such that the impingementcooling air introduced to the space between the dome heat shield 44 andthe dome 34 is forced to flow towards the respective gaps 56, 58 and isdischarged therefrom to form a cooling film along and substantiallyparallel to the hot side to cool the hot side of the respective innerand outer liner heat shields 46, 48. The exhausted impingement coolingair discharged from the respective gaps 56, 58 in the form of a coolingair film along the hot side of the respective inner and outer liner heatshields 46, 48 is substantially outside the primary zone 40 to reducethe NOx, CO and HC emission formation in the combustion gases. Thus,there is no low momentum effusion to be discharged from the exhaustedimpingement cooling air into the pocket of high fuel air ratio asindicated in circle A of FIG. 2, which helps to reduce the emissions andto maintain low gas temperature near the inner and outer liner heatshields 46 and 48 as discussed above with reference to FIG. 2.

A portion of exhausted impingement cooling air in the respective spacesbetween the inner liner 30 and the inner liner heat shield 46, andbetween the outer liner 32 and the outer liner heat shield 48 flowstowards the upstream end of the chamber 28 (as indicated by the arrowsshown in FIG. 3) to be discharged from the respective gaps 56, 58 tojoin the cooling air film along and substantially parallel to the hotside of the respective inner and outer liner heat shields 46, 48.Another portion of exhausted impingement cooling air in the respectivespaces between the inner liner 30 and the inner liner heat shield 46,and between the outer liner 32 and the outer liner heat shield 48 flowstowards the downstream section of the chamber 28 and discharged fromsuch spaces at respective downstream ends of the inner and outer linerheat shields 46, 48.

Optionally, the discharging apparatus may include a plurality of splashlouvers on the cold side at the downstream end of the respective innerand outer liner heat shields 46, 48, acting as a film starter for theportion of the exhausted impingement cooling air discharged from thedownstream end of the inner and outer liner heat shields 46, 48 to flowalong a downstream section of the respective inner and outer liners 30,32.

Optionally, the inner and outer liner heat shields 46, 48 may beprovided with two rows of small effusion hose 60, located downstream ofthe apertures 50 for introduction of dilution jet air, near thedownstream end of the inner and outer liner heat shields 46, 48.Therefore, a further portion of the exhausted impingement cooling air isdischarged from the effusion hose 60 and may enter the secondary zone 42(see FIG. 3).

The swirlers 38 are provided with swirl air passages 62 for introductionof air flows in a swirling flow into the chamber 28 to mix with the fuelejected by the fuel nozzles 36 for combustion. In this embodiment, theswirlers 38 each may include cooling air passages 64 for introducingcooling air into the combustor independent from the air flow introducedby the swirl air passages 62 of the swirlers 38. Each swirler 38 isconfigured as to provide a cooling air director 66 for directing thecooling air introduced from the cooling air passages 64 to generate acooling air film along and substantially parallel to a hot side of thedome heat shield 44 to cool the same.

In FIG. 3, the downstream section of the inner and outer liners 30, 32are made of single skin without the coverage of any heat shields. Aplurality of effusion holes 68 are defined in this downstream section ofthe inner and outer liners 30, 32 for directing cooling air therethroughand into the chamber 28 to effusion cool the downstream section of theinner and outer liners 30, 32, which is an efficient cooling method.

It is noted that in the combustor 26 of the reverse flow type, thedownstream section of the inner and outer liners 30, 32 are made ofcurved wall of large exit duct (LED) and small exit duct (SED) to turnthe flow. It is very cumbersome to manufacture curved heat shields anddifficult to minimize leakage between heat shields and the outerimpingement skin of the combustor inner and outer liners. The downstreamsection of the inner and outer liners substantially define the secondaryzone therebetween in which most of the fuel is already oxidized andthere is very low fuel air ratio, and therefore the effusion flowentering the effusion holes 68 and into the secondary zone will not helpto form NOx, CO and HC emission.

The hybrid cooling techniques for a gas turbine engine combustor, suchas using rich burn techniques, minimize cooling air entering in theprimary zone and thus high FAR can be maintained therein. Lack ofeffusion air in the primary zone with high FAR flame will not bestabilized locally near the combustor liners. This also helps tominimize thermal gradients in the structure members of the combustorliners by eliminating direct exposure to the flame in the primary zone,thus minimizing combustor liners buckling. The hybrid cooling techniquesimprove the reduction of NOx at high engine power conditions and CO/HCemission at low engine power conditions.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departing from the scope of the described subjectmatter. Modifications which fall within the scope of the describedsubject matter will be apparent to those skilled in the art, in light ofa review of this disclosure, and such modifications are intended to fallwithin the appended claims.

1. A combustor for a gas turbine engine comprising: inner and outerliners spaced apart from each other to define a chamber therebetweenincluding a primary zone adjacent an upstream end of the chamber and asecondary zone downstream of the primary zone, the upstream end beingclosed by a dome, a dome heat shield disposed within the chamber,adjacent and spaced apart from the dome, an inner liner heat shielddisposed within the chamber, adjacent and spaced apart from the innerliner, an outer liner heat shield disposed within the chamber, adjacentand spaced apart from the outer liner, the inner and outer liner heatshields extending from the upstream end of the chamber over the primaryzone and terminating at the secondary zone, the inner and outer linersand the inner and outer liner heat shields defining a plurality ofapertures therein for introducing dilution air jets into the chamber,the combustor including a plurality of impingement holes defined in thedome and inner and outer liners for directing impingement cooling airtherethrough and into the combustor to impinge on a cold side of therespective heat shields, and a discharging apparatus to direct exhaustedimpingement cooling air discharged from the cold side of the respectiveheat shields to flow along and substantially parallel to a hot side ofthe respective inner and outer liner heat shields, the combustor furtherincluding a plurality of effusion holes defined in a section of theinner and outer liners free of coverage by the respective inner andouter liner heat shields, the effusion holes directing effusion coolingair therethrough and into the combustor to effusion cool said section ofthe inner and outer liners.
 2. The combustor as defined in claim 1wherein the effusion holes defined in said section of the inner andouter liners are located downstream of the apertures for the dilutionair jets.
 3. The combustor as defined in claim 1 wherein the dome heatshield is disposed with respect to the inner and outer liner heatshields to define a respective gap between the inner liner heat shieldand the dome heat shield, and between the outer liner heat shield andthe dome heat shield to discharge a portion of the exhausted impingementcooling air in a cooling air film along the hot side of the respectiveinner and outer liner heat shield.
 4. The combustor as defined in claim1 wherein a swirler attached to the dome comprises cooling passages forintroducing cooling air into the combustor and a cooling air director togenerate a cooling air film along and substantially parallel to the hotside of the dome heat shield.
 5. The combustor as defined in claim 4wherein the cooling air introduced through the cooling passages in theswirler is independent from an air flow introduced by the swirler forcombustion.
 6. The combustor as defined in claim 1 wherein thedischarging apparatus comprises a plurality of splash louvers on thecold side at a downstream end of the respective inner and outer linerheat shields to act as a film starter for a portion of the exhaustedimpingement cooling air discharged from the downstream end of the innerand outer liner heat shields.
 7. The combustor as defined in claim 1wherein the inner and outer liner heat shields comprise a plurality ofeffusion holes at a downstream end thereof for discharging a portion ofthe exhausted impingement cooling air into the secondary zone of thechamber.
 8. The combustor as defined in claim 7 wherein the effusionholes defined in the inner and outer liner heat shields are locateddownstream of the apertures for the dilution air jets.
 9. The combustoras defined in claim 1 wherein the dome heat shield and the inner andouter liner heat shields comprise a plurality of pin fins projectingfrom the cold side of the respective dome heat shield and the inner andouter liner heat shields, for convection cooling the respective heatshields.
 10. A method for hybrid cooling a combustor of a gas turbineengine comprising steps of: a) providing heat shield panels in a chamberof the combustor to protect respective upstream sections of inner andouter liners of the combustor, the upstream sections of the inner andouter liners substantially covering a primary zone of the chamber; b)directing impingement cooling air for impingement cooling a cold side ofthe heat shield panels and then discharging a first portion of theimpingement cooling air along and substantially parallel to a hot sideof the heat shield panels and a second portion of the impingementcooling air into areas downstream of the heat shield panels, to therebyhave the discharged cooling air substantially outside of the primaryzone in order to reduce emission formation; and c) directing cooling airinto a downstream area of the chamber for effusion cooling of respectivedownstream sections of the inner and outer liners free of coverage bythe heat shield panels.
 11. The method defined in claim 10 furthercomprising a step of substantially restraining effusion cooling airwithin the chamber to an area downstream of dilution air jets, thedilution air jets being introduced into the chamber between the primaryzone and a secondary zone.
 12. The method defined in claim 10 whereinthe heat shield panels in step (a) covers a dome of the combustor. 13.The method defined in claim 12 comprising a step of using a swirlerattached to the dome to introduce cooling air independent from an airflow introduced by the swirler for combustion, to generate a coolingfilm along a hot side of a number of the heat shield panels covering thedome.
 14. The method defined in claim 12 wherein step b) includes usinggaps defined between a number of the heat shield panels covering thedome and a number of the heat shields covering the inner or outer linersto discharge the first portion of the impingement cooling air.